Resistor block and process of making



Patented Sept. 1, 1931 UNITED STATES STERN'E MORSE, OF CLEVELANDHEIGHTS, OHIO RESISTOR BLOCK. AND PROCESS OF MAKING No Drawing.

This invention relates to resistor blocks, and their method ofmanufacture and more particularly to blocks such as are used in heatingelements such as are described in my Patent No. 1,663,810, issued March27, 1928.

-An object of this invention is to produce graphite blocks ofhomogeneous composition, the blocks of the same size havingsubstantially uniform resistances.

A further object of this invention is to produce such graphite resistorblocks of substantially the same size and with surfaces which do notneed further finishing by grinding or machining.

A further object of this invention is to secure resistor blocks havingsurfaces such that the contact resistance is relatively sensitive toslight changes in pressure.

A further object of this invention is to produce such graphite resistorblocks of high density, and with considerable mechanical strength andheat conductivity.

Another object of this invention is to produce such blocks with aminimum. amount of 26 interstices between the individual particles ofgraphite to insure a minimum amount of oxidation when at hightemperature.

A further objectof the invention is the process for making graphiteresistor blocks 30 from artificial or natural pulverized graphite, orfrom such forms of amorphous carbon as-may be readily graphitized.

The artificial or natural graphite, which isusually powdered, ispreferably mixed with v a suitable binder, such as coal tar pitch, dis

solved in benzol or some other suitable solvent Which may bereadilyevaporated. Sufiicient a binder is preferably used to cause theindividual particles of the mass to readily and quickly adhere to eachother under moderately heavy ressures, thus avoiding the use ofcontinued heavy pressures, such as are customarily necessary when nobinder is used.

.The pulverized graphite is thoroughly mixed with the binder, preferablyuntil the consistancy 'is' that of fairly plastic mortar, "and thesolventof the binder is then evaporated in any suitable manner. Afterevapo- J 50. ration ofthe solvent, the'material is in the Applicationfiled September 24, 1927. Serial No. 221,819.

state of a very soft friable mass whichbreaks up easilyinto a powderconsisting of themdividual particles of graphite coated with a finecoating of the pitch binder. This material can be compressed in a slowacting press to form a firm mass, but cannot, under ordinary conditions,be advantageously pressed in a rapid, automatic press, such as a tabletpressing machine, since it cannot be made to feed properly. The materialalso contains so much air that the individual particles are held a artduring the instant when pressure is app ied, so that it is extremelydifficult, if not impossible, to form a firm mass by pressing' in arapid, automatic press. For the above reasons the material is pressed byrolls, or in a suitable slow acting press, with sufiic'ient pressure toform fairly firm cakes or sheets. These cakes or sheets'are then againbroken up and grained with very'low pressures in order to keep the sizeof the particles as large as possible so as to produce a coarse grainedmaterial. It is very desirable, in order to make the material feedproperly in an automatic press of the tablet machine type, that it bebrought to the physical condition Where it appears almost like sand,that is, the individual grains must be relatively compact sothat theycontain little air, and be of approximately the same size. The channelsbetween the individual grains being relatively large as a consequence ofthe graining process, there is time, even during the short instant whenfull pressure isapplied in a suitable tablet press, for the air to passout between the grains and for full coalescence of the grains to besecured in the subsequent final pressing to the desired shape. In thisgraining grinding must be avoided to prevent the formation of powder, asthe mass tends to break into fine powder very easily if it is ground,whereas coarse grains are desired.

This coarse-grained material is then classified in any suitable manner,such as by screening or pneumatic classifiers, into 'various grainsizes, so that the grains in one class will be of substantially the samesize with a View of pressing the material of each range of sir-esseparately. One good classification to profor example, a die, so thatwhen pressure is applied tothis loose material resistor blocks ofsubstantially the same weight and density will be formed. The resistorblocks, when so made from grains of substantially the same size, arequite uniform in character, so that they may be used interchangeably andform heating elements for use as set forth inmy copending application,Serial No. 86,413, filed February 26, 1926.

The material which passes the 40 mesh screen may be mixed with asuitable solvent to redissolve the binder and may be treated in the samemanner as above to yield coarsegrained material, or it may be added toanother batch of graphite and treated in the same manner as previouslydescribed.

The granules from each classification are then separately fed to asuitable press, such as, for example, a pellet or tablet formingmachine, and pressed with a considerable pressure to form a block of thedesired size and density.

The blocks are then mixed with an excess of powdered conductivematerial, such as graphite or carbon, and after the blocks and powderedmaterial have been thoroughly mixed together so as to form aconductivemass, the mass is packed into a suitable c011- tainer throughwhich an electric current may be passed.

The mass of tablets with the powdered conductive matter is made theresistor of a carbon resistance furnace with suitable graphiteelectrodes disposed at each end of the material. Preferably the furnaceis one of the type in which an adjustable pressure can be applied to allsides of the resistor mass, thus insuring that the mass of graphiteblocks and conductive material is compacted suificiently to readilyconduct the .current therethrough and allow easy control of the amountof energy supplied to the furnace.

A low voltage current is applied to the electrodes, and the mass isheated very slowly until the carbonizing temperature of the binder isreached. The mass is held at this temperature until carbonization of thehinder is substantially completed, and then the temperature is raised toabout 1800 F. and held there for several hours to insure completecarbonization of the binder. desired to have the blocks completelygraphitized so that they will be more durable and suitable for serviceat higher temperatures,

the temperature of the furnace can be raised If it be terial may beremoved from the furnace after i the carbonization of the binder isentirely completed.

If it be desired to have the resistor blocks .comp'osed completely ofgraphite, it may be preferable to use powdered petroleum coke as theoriginal material from which to form the resistor blocks.- When thismaterial is used, the process is carried on with the binder to form .thepreliminary cake from which the coarse-grained particles are formed, andwhich are classified and pressed as hereinbefore described. The blocksare mixed with powdered conductive material and heated in a furnacesimilar to the process above described, but graphitization is carried tocompletion to graphitize both the grains of the block and the binder.

- When the carbonaceous material is derived from the aliphatic series,such as from cellulose or ordinary coal, a graphite will be obtainedwhich is similar in crystallization to ordinary natural graphite, but ifit is obtained from the benzol series, as from petroleum coke, graphiteis obtained which differs from the graphite from the aliphatic series inits type of crystallization. The latter is somewhat preferable, as ityields a harder block of'higher resistance and also has the property ofbeing more refractory.

In order to secure a homogeneous mixture between the powdered graphiteand the binder, the coal tar pitch binder is preferably ground withbenzol, or other suitable solvent, in a ball mill until a smooth,even-flowing mixture is secured. One good grade of coal tar pitch isthat commonly known in the trade'as having a melting point of 175 F. Itis important that the coal tar pitch used be relatively low inso-called' fixed carbon, as such fixed carbon is relatively insoluble inthe ordinary solvents, and when present in appreciable quantities,seriously affects the adhesive quality of the binder.

If it is desired to graphitize the hinder, or to graphitize thecarbonaceous material when such carbonaceous material is used to i formthe blocks, it is preferable to incorporate a small percentage of asuitable catalyst,

such as iron oxide, alumina, silica, ferrous boroslllcate, or boron inany other form. The

presence of such catalyst materially assists 1 for this might cause theto be less stable.

It may be noted that by keeping the die and plunger of the press highlypolished, the resistor blocks will have good finished surfaces formaking electrical contact without the necessity of grinding ormachining.

The density and strength of the resistor blocks may, of course, beincreased with an increase of pressure in the tablet forming machine orother press that may be used. If the grained material to be pressed isheated and pressed hot, less pressure is necessaryto obtain blocks ofthe samecharacteristics, and the time of pressing may be reduced.

If it be desired to give the resistor blocks a higher resistance anon-carbonaceous binder such as clay may be used which may be mixed withany suitable liquid binder, such as a solution of water-glass or thelike, if the characteristics of the clay are such that an additionalliquid binder is desirable. When clay is used as a binder the processmay be carried on as above described except that in the process ofbaking the resistor blocks no attention need be paid to thecarbonlzation of the binder.

It may also be noted-that if the resistor electrical properties blocksare to be used at relatively low temperatures and relatively highresistances are desired as, for example, in the control of refrigeratingdevices, it may be unnecessary to even bake the blocks if they have beenformed with relatively high pressure. Preferably, however, they shouldbe heated suflicien tly to'insure removal of all of the volatile matter,and thereby prevent an adhesion of the resistorblocks to each otherduring use. This may also be prevented to some extent by the additionalcoating of these blocks with powdered graphite. It is usuallypreferable, however, when the resistor blocks are to be used withrelatively large currents or above normal temperatures to give them athorough baking and carbonize the binder.

Furthermore, it is to beunderstood that the particular compoundsdisclosed, and the procedure set forth, are presented for purposes ofexplanation and illustration, and that various equivalents can be usedand modifications of said procedure can be made without departipg frommy invention as defined in the appended claims.

What I claim is 1. The process of making carbmiaceous bodies whichcomprises mixing powdered carbonaceous material with a suitable binder,in solution, forming coarse solid granules of the carbonaceous materialof predetermined size, pressing such granules with a relatively greatpressure in a mold chamber having highly polished metal engagingsurfaces, mixing the bodies so formed with carbonaceous conductivematerial and heating the bodies and carbonaceous conductive material ina furnace to substantially carbonize the binder.

2. The process of making carbonaceous bodies, which comprises mixingpowdered carbonaceous material with a suitable binder in a volatilesolvent, evaporating the solvent from the mixed mass, pressing the drymass into cakes, breaking the cakes into solid granules of predeterminedsize, pressing said granules into blocks, mixing the bodies withconductive material, and heating said bodies and conductive material asthe resistor in a resistor furnace to substantially carbonize saidbinder.

3. The process of making carbonaceous bodies having predeterminedcharacteristics, which comprises mixing powdered carbonaceous materialwith a binder dissolved in a solvent to form a mass of plasticconsistency, evaporating the solvent from the mass, pressing the residueinto cakes, breaking the cakes into coarse solid granules, sorting thegranules into classes according to predetermined sizes, pressing apredetermined quantity of said granules of a predetermined size undergreat pressure into a body, mixing said body with carbonaceousconductive material, and baking said body and conductive material in afurnace to completely earbonize said binder and produce a homogeneousbody of predetermined characteristics.

4. The process of making carbonaceous bodies having predeterminedcharacteristics, which comprises mixing finely divided carbonaceousmaterial with a suitable binder, pressing the mixture into cakes,breaking the cakes into solid granules, sorting the granules intoclasses according to predetermined sizes, pressing equal predeterminedquantities of the granules of one class into bodies, and carbonizing thebinder by baking the bodies.

In testimony whereof I afiix my signature.

STERNE MORSE.

